1. Field of the Invention
The present invention relates to a method, device, and system for transmitting a supervisory optical signal.
2. Description of the Related Art
In recent years, information demand has rapidly increased in concert with the development in multimedia network, and an arterial optical transmission system integrating an information capacity is demanded to further increase a transmission capacity and form a flexible network. At present, wavelength division multiplexing (WDM) is the most effective technique responding to such a system demand, and the commercialization of WDM is now being pursued actively mainly in North America. A method of supervising a system adopting WDM is an indispensable function in operating the system. The present invention provides such a supervising method and its associated device suitable for a system adopting WDM.
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put into practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, an optical amplifier for amplifying an optical signal or signal light has been put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and a pumping unit for pumping (exciting) the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has been developed to amplify signal light having a wavelength band of 1.55 .mu.m where the loss in a silica fiber is minimum. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength band of 1.55 .mu.m can be obtained.
Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain main signal light (WDM signal light), which is output to an optical fiber transmission line. At a receiving end, the main signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data (a main signal) is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity by a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain of the optical amplifier which characteristic is represented by a gain deviation or gain tilt. For example, in a typical EDFA, it is known that a gain deviation is produced at wavelengths near 1.55 .mu.m. If the gain deviations in a plurality of cascaded EDFAs are accumulated, an optical SNR (signal-to-noise ratio) in a channel included in a low-gain band is degraded. Accordingly, to allow high-quality transmission, it is preferable to flatten the wavelength characteristic of gain of an optical amplifier.
On the other hand, to increase the number of WDM channels, it is effective to broaden the bandwidth of a gain band (a band where gain is generated) of an optical amplifier. For the purpose of such bandwidth broadening, there has been proposed a device including two optical amplifiers arranged in parallel, wherein one of the two optical amplifiers amplifies optical signals having wavelengths in a shorter-wavelength band and the other optical amplifier amplifies optical signals having wavelengths in a longer-wavelength band.
In any case, one optical amplifier or two optical amplifiers connected in parallel is/are provided in an optical repeater, and a plurality of such optical repeaters are arranged along an optical fiber transmission line.
To perform supervisory control of each optical repeater and the other system components, a supervisory optical signal is transmitted. For example, a supervisory optical signal for providing information on the number of WDM channels in operation is transmitted from a transmitting end to each optical repeater. In each optical repeater, a reference level to be used in ALC (automatic output level control) for maintaining an output level per channel constant is set according to the supervisory optical signal received. Some conventional methods for transmitting a supervisory optical signal will now be described with reference to FIGS. 1 to 3.
FIG. 1 is a block diagram of a conventional system for transmitting a supervisory optical signal. This system includes a first terminal device 2 as a transmitting end, a second terminal device 4 as a receiving end, an optical fiber transmission line 6 placed between the terminal devices 2 and 4, and a plurality of optical repeaters 8 arranged along the optical fiber transmission line 6.
The terminal device 2 includes an optical transmitter (TX) 10 for outputting an optical signal of one channel or WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths as main signal light, and an optical supervisory circuit (OSC) 12 for outputting a supervisory optical signal. It is now assumed that the optical transmitter 10 outputs main signal light obtained by WDM in a conventional band (referred to as "1.55 .mu.m band" or "C band") defined by the range of 1.53 to 1.56 .mu.m, provided that WDM is applied to this system. The main signal light output from the optical transmitter 10 and the supervisory optical signal output from the optical supervisory circuit 12 are multiplexed by a wavelength coupler 14 and then supplied to the optical fiber transmission line 6.
Each optical repeater 8 includes an optical amplifier 16 for amplifying the main signal light. The main signal light and the supervisory optical signal transmitted by the optical fiber transmission line 6 are separated from each other by a wavelength coupler 18. The main signal light from the wavelength coupler 18 is supplied to the optical amplifier 16, and the supervisory optical signal from the wavelength coupler 18 is supplied to an optical supervisory circuit 20. The optical supervisory circuit 20 regenerates a supervisory signal as an electrical signal according to the supervisory optical signal received to perform processing based on the supervisory signal and update the supervisory signal. The supervisory signal updated is converted into a supervisory optical signal, which is in turn supplied to a wavelength coupler 22. In the wavelength coupler 22, the supervisory optical signal updated by the optical supervisory circuit 20 and the main signal light amplified by the optical amplifier 16 are multiplexed to be output to the optical fiber transmission line 6. The processing based on the supervisory signal to be performed in the optical supervisory circuit 20 includes setting of a reference level to be used in ALC in the optical amplifier 16 according to the supervisory signal or updating the supervisory signal according to a monitored value of the gain of the optical amplifier 16, for example.
The second terminal device 4 includes a wavelength coupler 24 for demultiplexing the main signal light and the supervisory optical signal transmitted by the optical fiber transmission line 6, an optical receiver (RX) 26 for receiving the main signal light from the wavelength coupler 24, and an optical supervisory circuit 28 for receiving the supervisory optical signal from the wavelength coupler 24.
In the system shown in FIG. 1, the optical supervisory circuit 20 is arranged in parallel to the optical amplifier 16 in each optical repeater 8. Accordingly, the supervisory optical signal is sequentially updated as required and transmitted toward the receiving end.
FIG. 2 is a block diagram of a system that may be proposed from the prior art shown in FIG. 1 for the purpose of bandwidth broadening. In this system, the optical transmitter 10 outputs first main signal light obtained by WDM in the C band and second main signal light obtained by WDM in a long wavelength band (referred to as "1.58 .mu.m band" or "L band") defined by the range of 1.57 to 1.60 .mu.m. The first and second main signal lights output from the optical transmitter 10 and the supervisory optical signal output from the optical supervisory circuit 12 are multiplexed by the wavelength coupler 14 and then output to the optical fiber transmission line 6.
Each optical repeater 8 includes an optical amplifier 16(#1) for the first main signal light and an optical amplifier 16(#2) for the second main signal light. The two optical amplifiers 16(#1) and 16(#2) are arranged in parallel. To connect the two optical amplifiers 16(#1) and 16(#2) in parallel, each optical repeater 8 further includes an optical demultiplexer 30 for demultiplexing the first and second main signal lights to respectively supply the first and second main signal lights to the optical amplifiers 16(#1) and 16(#2), and an optical multiplexer 32 for multiplexing the first and second main signal lights respectively amplified by the optical amplifiers 16(#1) and 16(#2). The wavelength couplers 18 and 22 are provided outside of the optical demultiplexer 30 and the optical multiplexer 32, respectively, thereby allowing processing of the supervisory optical signal in the optical supervisory circuit 20.
FIG. 3 is a block diagram of another system that may be proposed from the prior art shown in FIG. 1 for the purpose of bandwidth broadening. In this system, the optical transmitter 10 outputs first main signal light in the C band and second main signal light in the L band as in the system shown in FIG. 2. The optical supervisory circuit 12 outputs first and second supervisory optical signals respectively related to the C band and the L band.
Each optical repeater 8 is similar to that in the system shown in FIG. 2 in the point that it includes optical amplifiers 16(#1) and 16(#2), an optical demultiplexer 30, and an optical multiplexer 32. In each optical repeater 8 shown in FIG. 3, an optical supervisory circuit 20(#1) for the first supervisory optical signal is connected in parallel to the optical amplifier 16(#1), and an optical supervisory circuit 20(#2) for the second supervisory optical signal is connected in parallel to the optical amplifier 16(#2). To configure such parallel connection, wavelength couplers 18(#1) and 22(#1) for the optical amplifier 16(#1) and the optical supervisory circuit 20(#1) are provided so as to interpose the optical amplifier 16(#1), and wavelength couplers 18(#2) and 22(#2) for the optical amplifier 16(#2) and the optical supervisory circuit 20(#2) are provided so as to interpose the optical amplifier 16(#2).
In general, an optical amplifier has its inherent noise characteristic and gain efficiency (gain conversion efficiency: the ratio of output optical power to pumping energy). The noise characteristic and gain efficiency of an optical amplifier are substantially determined by a band to which the optical amplifier is applied. For example, an EDFA for the C band has a noise characteristic and a gain efficiency both superior to those of an EDFA for the L band.
In the system shown in FIG. 2, a loss by the wavelength coupler 18 for branching off the supervisory optical signal in each optical repeater 8 is given uniformly to the optical amplifiers 16(#1) and 16(#2), so that there is a possibility that the noise characteristic and gain efficiency of the optical amplifier 16(#1) or 16(#2) may be unduly degraded. Similarly also in the system shown in FIG. 3, substantially equal losses by the wavelength couplers 18(#1) and 18(#2) are given to the optical amplifiers 16(#1) and 16(#2), respectively, so that there is a possibility of undue degradation in the noise characteristic and gain efficiency of the optical amplifier 16(#1) or 16(#2). In general, a degradation in noise characteristic and gain efficiency of an optical amplifier causes a degradation in transmission quality of a main signal.